A case of gonadoblastoma on one side in streak gonads in a phenotypic female with chromosomal 46,XY pattern is described. The histopathological resemblance between gonadoblastoma and sex cord tumor with annular tubules is discussed. The high serum testosterone level, being unrelated to the administration of human chorionic gonadotropin is valuable for the diagnosis of gonadoblastoma in an individual with dysgenetic gonads. Gonadoblastoma is frequently very small in size and is located in ectopic regions, so that laparotomy with biopsy has been stressed. The hormonal supplement for a postgonadectomized girl is also discussed.